This invention relates to photomultiplier tubes and particularly to an evaporator support assembly for such a tube.
In the manufacturing of photomultiplier tubes, the reproducibility of uniform, high sensitivity photocathodes produced by the vapor deposition of at least one alkali metal on a suitable substrate is desirable. In the design of most photomultiplier tubes, a great deal of thought is given to the electron optics, dynode cage design, photocathode processing and parts cleaning procedures. However, the mechanics of tube assembly such as the insertion of one or more alkali evaporators, comprising an alkali metal chromate enclosed in a thin tantalum retainer seems, in most cases, to be left to chance, i.e., the alkali evaporators are placed in the tube wherever a convenient space for them may be found. Typically, the alkali evaporators are placed within the tube with little of the precision given to the placement of the other tube elements described above. As a result, the sensitivity of the photocathodes varies in uniformity and stability, due in large part to the non-precision of the evaporator placement.
It is thus desirable to precisely position the alkali evaporators within the tube so that they are mechanically and electrically substantially identically located from tube to tube in order to consistently reproduce high sensitivity photocathodes.
U.S. Pat. No. 3,753,023 to Sommer, issued Aug. 14, 1973, shows a typical photomultiplier tube in which the alkali evaporators are suspended adjacent to the dynode structure. The alkali evaporators are shown to be substantially unsupported and unrestricted within the tube. The electrical resistance of the evaporators, which comprises not only the resistance of the tantalum retainers but also the resistance of the attached lead wires connecting the ends of the evaporator retainers to the leads in the stem of the tube, varies from tube to tube. This variation is undesirable because the current at which the alkali vapors are emitted from the evaporators will also vary from tube to tube, thus creating processing differences between the tubes. These differences are undesirable and often result in substantial photocathode sensitivity variations between tubes processed by the same cathode processing operator.
An attempt to improve the cathode processing of tubes using resistance heated alkali metal evaporators is described in the copending U.S. patent application Ser. No. 025,559, abandoned, filed on Mar. 30, 1979, by Butterwick and assigned to the same assignee as the present invention and incorporated by reference herein. The Butterwick patent application describes a generator which contains alkali sources. The generator is mounted in the photomultiplier tube adjacent to the dynode assembly. The generator, however, serves as a receptacle for trapping particles of alkali metal powder and for directing the alkali metal evaporation substantially toward the cathode surface and does not precisely locate the evaporators mechanically or electrically within the tube. That is, the generator described in the copending patent application does not provide a structure that offers reproducible evaporation currents from tube to tube.